You hate to see this. Happened to all of us. You are right, if you went all in I don't think he would have called with pair of 3's. I don't really love the idea of going all in preflop since while KK is a very good hand, you can't know if he has pair of 3's or aces. Since you have a good stack and are in a big tournament, maybe a bigger bet preflop would have been more suited. If he called the bigger bet and go all in after that flop, the chances he has trips is pretty high. After a big bet from you, he has to consider you have a big pair like kings, but with 3 5 10 flop he still went all in. Hard pill to swallow and difficult fold, I don't think I could have gotten myself to fold there either, but it is what it is and with 50k stack, it's the smart move I believe.
I lost a crazy hand with pair of 3's. The flop was K K 3. Someone went all in, I thought man poor guy, gets king trips and still lose. It shows his hand, K Q. Poor guy. I'm counting my chips. I'm not even paying atention to the table anymore. The turn comes, then the river, and then I get the message saying congratulations for this position in the tournament and I'm like no way, this can't be. Turns out the river was a Q, so that was that for me lol. Sometimes we win big, sometimes we lose big.